I 1 NASACR-813 >
SOME FASA CONTRIBUTIONS TO THE USE OF PLASMA J E T TECHNOLOGY
IN CHEMICAL PROCESSING
Bi*Howard C. Ludwig i”
-
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Prepared under Contract N&. NASw-1177 b y / WESTINGHOUSE ELECTRIC COR~ORATION
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for
NATIONAL AERONAUT ICs AND SPACE ADM IN ISTRATION
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SCME NASA CONTRIBUTIONS TO THE USE OF PLASMA JET TECHNOLOGY IN CHEMICAL PROCESSING
Howard C. Ludwig Atomic &Molecular Sciences
Westinghouse Research Laboratories
ABSTRACT
Due mainly to the impetus of the Space program's specifications
for testing materials under conditions of air atmosphere reentry simulation
and for space travel and guidance control, space-oriented Federal agencies
are concerned with the technology of high temperature air heating and other 1
plasma producing devices.
In the fulfillment of these tasks it is always desirable that the
acquired technology provides spin-off applications which contribute to the
nation's industrial progress.
It is the purpose of this writing to point out the economic and
technical potentialities of some plasma producing devices in chemical
processing, where it is felt the greatest impact on economic welfare may be
realized.
iii
CHAPTER 1. Potentialities and Requirements. . . . . . . . . . . . . 1
EnergyLevels. . . . . . . . . . . . . . . . . . 3
Arc Physics. . . . . . . . . . . . . . . . e . . . e . 4
CHAPTER 2. The Design of Plasma-Jet Devices . . . . . . . . . . . . 8
Core-Flow Theory . . . , e . . . . . . . . . . . . . , 12
Volumetric Heat Addition Theory - Stine-Watson Model.,.... . . . . . . . . . . . . . . . . . . . 13 Energy Exchange at the Arc Terminals . , . . , . . . . 16
CHAPTEK 3. The State of Technological Advancement . . . , . , . . 19
Problem Areas. , , e , . . . , . , , . . , . , . 22
REFERENCES. . . . . , . . , . . . . . . . . . . . . . . 26
V
CHAPTER 1
Potentialities and Requirements
A technology previously limited mostly to scientific investigation
but now on the verge of widespread practical application may enrich the economy
both by reducing the cost of existing coxnodities and by creating new goods.
A new industry may be established to supply the apparatus needed for the
various new processes, but unless new products are developed by the technology,
its growth will of necessity be somewhat at the expense of established
equipment industries.
Many intermediate and finished items will be influenced by any
reduction in cost or improvement in attributes brought about by use of the plasma
jet process. This is particularly true of plastics, synthetic resins, and
fertilizers.
in the chemical and petro-chemical industries.
presently highly speculative processes may eventually prove to be successful.
Such areas would include petroleum refining, improved man-made fibers, and soaps
and detergents.
The largest industrial potential for plasma jet technology is
There is a possibility that some
One interesting possibility is cheap on-site manufacture of fertilizer
in the world's under-developed countries.
the fixation of nitrogen might lead to this, particularly in areas where water
Plasma chemistry developments on
power is available to produce electricity.
There is also considerable experimental activity in other areas,
but economic evaluations must await future developments. Plasma jets may
figure in the production of cyanogen, hydrogen, carbon black, pyrographite,
fluorocarbons, and uranium carbide.
several industries may experience growth as a result of future
plasma jet innovations.
at an annual rate of $1 million would show an expansion. Application of the
plasma jet to produce acetylene is approaching economic feasibility.
market price of acetylene is currently over $150 million a year.
welding, facing, and spraying industries part of which are already using
plasma jets operate at an annual rate of $30 to 40 million.
The industry now manufacturing plasma jet devices
The
The cutting,
It is possible
that the plasma jet will penetrate the chemical synthesis and processing
industry which is at a $2 billion rate.
Based on past experiences with new technologies, it seems highly
likely that beyond the next decade entirely new chemical products will appear
as a direct result of the continuous very high temperatures that can be achieved
with the plasma technique. It is, of course, impossible to do more than
conjecture in regard to the nature of the products and it would be even more
imprudent to attempt to gauge the economic impact.
2
Energy Levels
The thermal energy levels attained by the flowing medium in a plasma
jet place its atoms and molecules in electronically excited and ionized states.
This increases the free valencies and chemical reactivity of the participating
materials. The successful development of continuously operating plasma jet
devices of production size would enable industry to adopt new processes which
have looked promising when investigated cm a small-scale laboratory basis.
Plasma jet devices are used now for spray coating, cutting and welding
at power loadings to 50 kW, but may have a greater economic impact on the
chemical industry, where the required power loading is in the megawatt range.
Space programs have prompted advances in the design of plasma jet hardware
in an ever widening range of power input capacity. Plasma jet devices have
operated on a continuous basis in the multimegawatt range, Fig. 1, and as
low as 50 watts, Fig. 2. That highly powered plasma jet devices developed
for wind tunnel applications are adaptable to chemical processing has been
shown by recent studies of acetylene production which indicated improved
yields. (1)
3
Arc Physics
High pressure arc dischargeo have been long recognized as a source
of heat and radiation reaching temperatures of 5000 to 50,0OO0K which are as
high and higher than those attainable in combustion flames.
and controlled at pressures ranging from tens of mm Hg to about 100 atmospheres
and in convected flow velocities from a few meters per second up to the hypersonic
range.
200,000 Btu/lb. in sane cases.
Arcs are maintained -
Heat contents of arc-heated gas can be developed to values approaching
Electrical current flows in an arc through potential drops across
the terminals in three regions: the cathode space, positive column, and anode
space.
atom ions, although molecular ions of negative or positive charge, less important
for charge transportation but significant to chemical reaction, may be present
The principal charge carriers in the arc are free electron8 and positive
as a consequence of the thermal ionization of certain chemical compounds.
expended electrical energy is transfotmed into heat and radiation through Joule
heating by the electrical conduction process of the arc, which is characterized
by its composition, its pressure, and the power loading. An energy balance is
maintained by dissipation of the input energy through heat conduction, convection
and radiation.
dissipation proce~~ses.
The
Thermal equilibrium i o continually disturbed by the energy
The arc coluppn consists of a heavily radiating mixture
4
of free electrons, positive ions, negative ions and highly excited atoms,
constituting what is called plasma, through which the energy exchange processes
develop.
An important consideration in the energy balance is the radiative
process, which can result in severe energy loss to the walls of the arc-confining
space.
energy expended in a plasma jet device particularly as the power input and
operating pressure increase,
plasma when the plasma constituents lose energy in some of the exchange processes.
The nature of the radiation, which depends on the energized medium, pressure, and
energy level attained, is characterized by emission, band, line or continuous
spectra.
result of this energy loss.
repeated absorbtion and re-emission until it develops in the outer zones of
the arc.
absorption and re-emission by excited atoms are produced more effectively, by
increased emission developed within the arc column.
low energy level band spectra are emitted in the outer low temperature zones of
the arc column.
excited atoms are emitted.
Radiation may constitute a large fraction of the transformed electrical
Radiation is emitted by the thermally energized
Emitted radiation quanta have a cooling effect on the plasma as a
The transport of radiation may be enhanced by
Certain wavelengths of the radiation which are capable of repeated
In arcs of molecular gases,
In the regions of higher temperature, the spectral lines of
5
0 At temperatures above 10,000 K, intense ion lines of the gas are
also emitted.
important to know in which regions of the arc the greatest energy is converted,
The operating power levels, the properties of the arc plasma, (composition, . pressure, etc.), and the camposition and design of electrodes all determine
the radiative output and spectral characteristics of the discharge.
Therefore to understand the total radiation behavior it is 4
For
example, in the carbon arc, over 80% of the total radiation is emitted by
the glowing anode. In the case of mercury or inert-gas high-pressure
arcs between tungsten electrodes, the radiation from either electrode is small
in comparison to that emitted by the arc column. In a xenon high pressure arc,
most of the radiation comes from a small spherically shaped zone situated
immediately in front of the cathode. In the Gerdien arc, burning in an internally L water-sprayed plasma jet device, an enormous energy density of 11,000 kW/cm
is obtained. ( 2 )
of emitting centers consisting of excited atoms and ions of hydrogen and oxygen
at a temperature of 50,0OO0K. In contrast, large fractions of the power input
can be transformed into radiation by operation at high pressure and high power
loading when using certain gases.
variation of power radiated per unit of thermally energized air (N2 and 02) as a
The radiative output is extremely low because of the small number
For example, Kivel and Bailey(3) show the
6
function of enthalpy and pressure, Fig. 3,
In h%gh radiative output arcs, the radiation, unless absorbed or
recaptured by properly introduced reactants, is absorbed by the cooled walls of
the arc-confining space. The importance of the high radiative output from arcs
used in highly powered plasma jet devices is recognized by NASA and is presently
under investigation.
profitably utilized in certain chemical reactions.
jet devices for a particular chemical process, depends heavily on the specific
knowledge of the plasma chemistry of participating reactants.
A s will be discussed later, the emitted radiation can be
Therefore, design of plasma
7
CHAPTER 2
The Design of Plasma-Jet Devices
In the design of plasma jet devices, two important guidelines must
(1) the power loading required must be determined, to attain be considered:
the enthalpy level and temperatures necessary to achieve the particular function
(thrust for propulsion, cutting, welding or dissociation in a chemical process)
and (2) the heat transfer capacity of the heat exchanging system must be sufficient
to handle the losses encountered at the required power loading. The engineering
of the optimum design requires; (1) adequate knowledge of the thermodynamics and
chemistry of the plasma medium, (2) the physics of the arc discharge and (3) the
dynamics of the plasma flow. All of this knowledge is culminated in a
theoretical model that should specify a conservation of energy equation which
includes a quantitative measure of each mode of energy dissipation, namely
radiation, convection and conduction. Having all of the information necessary,
one could predict the operating characteristics of a plasma jet device designed
around the theoretical model. Unfortunately, adequate knowledge for determining
optimum designs is not available, nor are arc discharges and practical conceptions
of hardware configurations simple enough in geometry to undergo calculation in a
rigorous manner. At best, models are simplified so that some calculations may
be made to serve as a phenanrenological guide to the analysis of the energy
transfer processes involved.
8
In general, the arc discharge of a plasma jet device can operate in
either an unconstricted or constricted configuration. (4)
configuration is one in which the arc is free-burning.
magnetic forces do not confine the arc to the extent of altering the shape or
the cross sectional dimension of the arc column, The constricted configuration
is one in which the arc is confined in a specified manner, The constricted
configuration is more realistic in practical design but the special nature
of the constricting forces in each case makes it difficult to develop an
all encompassing theoretical model. Some of the various forms of cnconstricted
and constricted arc configurations are schematically illustrated in Figs. 4 and 5
respectively. Fig. 6 illustrates a special form of toroidal shaped electrodes on
and between which the arc is rotated.
the current-carrying arc column with an external magnetic field.
NASA Langley contracted development--was used in the plasma jet device, shown
in the photograph of Fig. 1 and the illustration of Fig. 6. It was originally
The unconstricted
Solid walls or fluid and
'
The arc rotation is achieved by crossing
This design--a
developed for wind tunnels but more recently was applied to chemical processing.
In the electrode design, the arc is not severely constricted by solid walls but
by the radial magnetic field interaction with arc current,
results in a force causing the arc to assume a circular motion.
constricted by self-magnetic field developed by the flow of arc current (pinch
effect) and by the convection losses due to the mixing action of the arc as it
rotates in the flowing gas.
The interaction
The arc is
9
A special note is taken of the plasma jet device design shown in Fig. 7
(5) using two coaxial tubular electrodes separated by a feed stock inlet chamber.
One electrode has an opening for exhausting the arc plasma.
surrounding the rear electrode magnetically rotates the arc.
at unusually high voltages, in excess of 2000 volts.
above high voltage plasma jet device, (Fig. 7), was derived chiefly from
experimental data obtained from three models of different arc power capacities.
Although approximate, the scaling factor N, defined as the ratio of linear
dimensions in the scale model to those of another device nominally rated at two
megawatts input, serves well as a guide to the selection and operating conditions
for a similar device to achieve a required enthalpy.
efficient air enthalpy and arc chamber pressure of an N scale device are equal
to those of the basic N P 1 device providing the air flow rates per unit area are
the same,
will equal those of the basic device multiplied by N and N , respectively.
A field coil
This device operates
A scaling law for the
It was found that the
Under these conditions, the scale model arc voltage and power 2
In reality, all arcs operating in a forced flow field are constricted.
Therefore a theoretical model based on the unconstricted arc configuration is in
error to a degree depending on the strength of the force field.
In principle, any gas or vapor can be utilized as a feed stock in a
plasma jet device.
the pressure and enthalpy are specified, the amount of energy vested in excitation,
ionization and dissociation for a given feed stock can be calculated from a Mollier
chart which relates equilibrium values of enthalpy to pressure, temperature and
The selection is baaed on the function to be performed. If
10
enthropy. The Mollier chart for hydrogen(6' is shown in Fig. 8.
Another important consideration is the behavior of the plasma flow,
especially if a certain velocity is important to the function.
after being heated, may pass through a constricting throat to develop a higher
velocity. If the flow variables change only with respect to the axial flow
direction, the flow can be mathematically treated as one dimensional, which
represents only an approximation to most actual cases.
from gas to structure of the plasma jet device and the presence of a boundary
layer near the confirming walls destroy the ideal flow situation.
The feed stock,
The obvious heat transfer
To analyze the energy-transfer mechanisms occurring in plasma jet
devices, several models have been proposed.
the feed stock to be heated uniformly, in which case the mechanism of heating is
not important. A model based on uniform heating of the feed stock can only
roughly describe the operation of plasma jet devices since heating and flow
nonuniformities exist.
constricted arcs from which an adequate analysis of the energy transfer mechanism
can be made; the core flow theory") and the Stine-Watson or volumetric heat
addition theory. (8)
The most obvious approach assumes
Thus far, there have been only two proposed models for
These two theories are described below.
11
Core-Flow Theory
A basic feature of the core-flow model is to consider the fluid through
the arc constricted zone as two flames concentrically located around a thin core *
flow likened to B thin wire.
that portion of the total flow which is electrically conducting and in which
Referring to Fig. 9, the core flow is defined as
energy due to Joule heating is liberated radially through the core surface to
the "inner flow. 'I Longitudinal transport of energy through the central core is
considered negligible canpared to the radial transport through the core surface.
The inner flow is in turn, surrounded by the outer flow of relatively cold gas,
The purpose of the thin stagnate arc is to produce thermal energy and transport
it to the inner flow is thus heated directly by the arc and carries the main
part of the energy. The relatively cool outer flow, not heated directly by the
arc, receives radiative energy from the arc and energy by thermal conduction from
other parts of the plasma jet device structure.
electric field strength, the operating pressure, current and boundary condttions,
the radial arc temperature distribution and the enthalpy can be calculated. The
basic equations which describe the behavior of a fully developed laminar plasma
column are given by:
Knowing the plasma column
2 Ld (r k - dT +#E )= Pr r dr dr
12
and
for boundary conditions
- P 0 at r = 0 and T = Twall at r = r dr wall
whereeis the local electrical conductivity, E is the plasma column field
strength, r is the radial distance from the column axis, k is the local thermal
conductivity, T is local temperature, Pr is local column volumetric-radiation
and I is the current.
The assumption of negligible mass flux transported through the thin
current conducting core is not strictly valid. Even though the ma88 flux may
be small, the axial temperature of the arc column can be sufficiently so high
a8 to contain a large amount of energy.
plasma is low and the conduction of heat energy to inner flow is low, then the
axial convection transport of energy through the core would not be considered
negligible.
to explain the axial enthalpy flux through the conducting core.
of energy at the arc terminals are not considered.
If the thermal conductivity of the core
Probably the basic drawback of the core flow model is the inadequacy
Also, the losses
Volumetric Heat Addition Theory - Stine-Watson Model In contrast to the core flow theory, the arc can be assumed to fill a
large part of the constricting zone as shown in Fig. 10. The Stine-Watson model
13
accounts for an axial convection term, considered negligible in the core flow
model. The energy equation, after simplification is given by.
where f gas density
Vz gas velocity in axial direction
h enthalpy
E electric field
r radius of constricting region
arc radius rO
and S = heat conduction function Kdt.
K is the thermal conductivity of the arc discharge plasma.
assumed constant. The term is the enthalpy gradient. The axial heat
J The value c V z is
2 d z conduction term2 S/Jz* is neglected because of its small contribution. The
radial conduction term must be retained, representing a losr to the constructing
wall. This energy equation is solved for the enthalpy h(r,z) by assuming that
14
the enthalpy of the gas can be simply related as h = (Cp/K) S where C is the
specific heat at constant pressure, and that the electrical conductivity (a) P AS where A rationalizes electrical and thermal conduction, and has the unit arc
P
voltage. In its simplified form, the conservation equation is solved to give
w c where z t 2; w is weight flow rate of gas; and C is a constant depending on
the units of enthalpy, Jo is a Bessel function of zero order, 0
The important function h (r,z) obtained by the above equation permits
the calculation of several important engineering quantities such as the local
heat loss from the arc, the voltage gradient, radial enthalpy as a function of
axial position, the total heat loss from the arc surface, and the efficiency with
which the electrical power is transferred to the working gas. An important
prediction of the Stine-Watson model is that the radially averaged enthalpy
leaving the plasma colunm is inversely proportional to the arc radius and
proportional to an expressed function
-11.5 z 1/2 (1 - e -I z
0
Agreement is given between the Stine-Watson theory(7) and experimental data(8) as
shown in Figures 11 and 12.
15
Failure of either the core flow or the volumetric heat addition theory
in regions near the cathode and anode region is to be expected. Both theories
can be fitted to certain experimental results by appropriate choice of boundary
conditions.
energy transfer mechanisms of conduction, convection and rotation as new designs
of plasma jet devices are developed and as additional experimental data are
obtained.
Both models will continue to change, to more adequately treat the
The design of a plasma jet device as shown in photograph of Fig. 1 and
in the schematic drawing of Fig. 6, lends itself to a simple model of uniformly
heated feed stock because of the mixing action of the magnetically driven arc.
However the influence of the radiative mode of energy exchange introduces
complications which make it difficult to apply such a simple theory,
output from arcs in some cases cannot be neglected because of appreciable
contribution to the energy balance.
The radiative
Energy Exchange at the Arc Terminals
The current of an arc discharge flows along an electrically conducting
gas path terminating at the electrodes.
plasma jet devices, the electrical conductivity is governed by the plasma
temperature.
a problem because the temperature must fall from a high value in the plasma column
In the thermal arc, the heating source for
The electrical connection of the arc to the electrodes presents
16
to a temperature at the electrode surface lower than its melting temperature.
The electrodes, theref ore, must be cooled to prevent melting and vaporization.
At high power loadings the problem of protecting the electrodes fram severe damage
not only requires carefully designed cooling system but also a careful
consideration of the heat exchanging area over which the steep temperature gradient
occurs. It has been shown that the rapid rotation of the arc by the force
introduced by an external magnetic field permits larger power loadings without
severe damage to the electrodes.
operated to accept heat fluxes at the electrode surfaces of up to 20 million 2 Btu/ft. hr.").
has been found necessary to adhere to very stringent heat exchanger design
principles, to avoid bulk water boiling in the electrode cooling water passages.
The evenly spaced water passages must be very close to the electrode surface
exposed to the heat flux and cooling water velocities of 100 ft. per second
were required. The identical design for both electrodes permits arcs to be
operated with direct or alternating current as shown in Fig. 7.
configurations illustrated in Figs. 4 and 5 can only be used with direct current
because of an inequality in the heat flux occurring at the anode and cathode
terminals of the arc.
Plasma jet devices have been built and
In achieving the extremely high electrode cooling capacities, it
Some electrode
The mixing actfon produced by magnetically driven arcs helps to
uniformly mix the feed stock and provide a relatively uniform temperature profile.
17
A uniform radial temperature profile offers better control for heating reactants
in a chemical process.
devices at higher power loadings to develop higher enthalpies at higher feed stock
pressures.
multiphase alternating current provide promising design conditions for application
to chemical processing e
A magnetic arc drive allows one to operate plasma jet
Toroidal-shaped water-cooled copper electrodes operating with
18
CHAPTER 3
The State of Technological Advancement
The advancement of plasma jet technology in recent years has been
accelerated by the impetus provided by NASA.
engines and material testing for atmospheric re-entry have contributed
importantly to plasma jet technology that is equally applicable to industrial
needs.
employed in wind tunnels and the whispering small plasma jet engines designed
for satellite attitude-control.
Its interests in space propulsion
Our economy can benefit both from the roaring multi-megawatt arc heaters I
Some research and development efforts by chemical producing companies
already have advanced beyond small laboratory sized investigations to production-
scaled developmental programs.
in a recent NASA publication. (lo)
in hundreds of specific endothermic reactions.
Progress in plasma jet technology was discussed
The chemical industry may find it helpful
Among the more promising reactions investigated is the production of
The research and acetylene from natural gas (methane) by plasma jet reaction.
development on a 10-megawatt plasma jet device designed for arc heating air for
a wind tunnel installation at Langley Research Center has contribured importantly
to the technology of plasma generation in a power capacity suited for production
chemical processing. Plasma jet devices of the Langley wind tunnel type, as
19
shown in Fig, 1, have been built for the power loading range of 3 to 20 megawatts
which is sufficient for full scale chemical production,
jet of the toroidal shaped electrode tip with magnetically driven arc was
developed to withstand the severe operating conditions imposed by continuous-duty
service in a chemical processing application.
A 3 megawatt plasma
A recently published report describes the results obtained for the
production of acetylene from methane.
of applying the rotating arc principle
obtained by considering the high temperature characteristics of the system
C + 2 H2 starting with methane (CH4),
as a function of temperature is shown in Fig. 13.
concentration (9 mole percent) appears at 3600 degrees K.
arc, the methane experience an average temperature of about 7000 K and is
therefore completely dissociated.
is not arced in passing through the arc chamber to the downstream zoneo
mixing action promotes an efficient utilization of the energy of the arc.
Energy conversion rates of 2,64 kw hr./lb. (I1) have been obtained in
comparison to 4.5 to 6.0 kw hr./lb. reported for other electric arc processes.
As exemplified by the cited results, a continuing effort on the part of chemical
producers in their individual vested interests and electrical hardware
manufacturers promisee to yield a profitable atmosphere in a new technological
activity.
Some indication of the feasibility
to chemical processing may be
The composition of the equilibrium mixture
The maximum acetylene (C2 H2)
In passing through the 0
This highly energized gas mixes with gas which
The
20
Plasma jet devices designed for power consumption in the range of tens
of watts to 30 kW have been proposed by NASA for use as propulsion engines to
provide the main thrust and for attitude control in space travel.
of the requirements of long time service, low weight and stable operation in these
devices, many improvements in design and materials have been made which are being
reflected in plasma jet hardware designed for industrial application. Regenerative
heat exchanging and radiation cooling principles incorporated in the design of NASA
controlled developments of 2 kW and 30 kW plasma jet devices are shown in
Figs. 14 and 15.
boron nitride and the metals tungsten and molybdenum. The pattern graph of Fig. 16
shows the measured isotherm and heat flow paths for the 30 kW plasma jet device.
High-power thrustor (> 100 kW) developments by NASA are now in initial stages.
As a consequence
These designs require the use of refractory insulation such as
Of equal importance to industrial development of plasma jets is the
basic information about feed stock materials (I4) documented by NASA and the
Air Force, Navy, and Army. In addition to performance data on the parameters of
plasma jet operating conditions, the thermodynamic properties(6) heretofore not
available for many gases and materials have been developed over a temperature
range up to 100,OOO°K.
the Ames and Langley Research Centers have resulted in improved understanding
of this mechanism. The decrease in arc rotational velocity with increasing arc
chamber pressure or decreasing field strength was satisfactorily explained by a
concept based on the idea that the arc agts as though it were a solid aerodynamic
Research studies of magnetic field--arc interaction at
21
body having drag.
and the resultant equilibrium velocity is that velocity at which the drag force
on the arc equals the electromagnetic driving force.
arc velocity with increasing arc chamber pressure is caused by the increased drag
on the arc.
to the development of plasma jet devices for chemical processing.
The arc is driven by the electromagnetic force of the field
The effect of decreasing
The results of this study are considered of paramount importance
The field of high temperature instrumentation is richer because of
the contributions of all NASA Research Centers. Outstanding among the instruments
is a probe developed by the Lewis Research Center for the simultaneous measurement
of static and impact pressures'lo'in the cross section of plasma jet devices.
Problem Areas
Technological advancements in plasma jet devices made in the last 10
years have to a great extent eliminated severe problems of heat exchanging and
material failure. However, where high enthalpy, high pressure operations are
required, new concepts in convected cooling of electrodes and associated hardware
are needed. A marked improvement in heat exchange has been obtained by the
application of magnetic fields crossing the gas and arc current flow.
action developed by the magnetically driven arc alleviates the problem but does
eliminate it.
The mixing
A barrier in the enthalpy-pressure parameter remains under some
22
operating conditions. At pressures above 50 atmospheres the maximum attainable
enthalpy is likely limited by the heat flux of forced convection water cooling
and the increased radiation output.
mechanisms such as adsorption and reemission, heat transfer loading of the cooled
Primarily because of radiation transport
walls and electrodes exceeds their heat exchanging capabilities apparently due to
radiation penetration to the confining walls.
are needed to assist in the understanding of the radiative energy transfer
mechanisms as they pertain to plasma jet devices.
Systematic experimental studies
Absorption of radiation by injected reactants holds promise for
advancements in chemical production techniques.
to operation level of a plasma jet device, the radiation developed in the arc may
be utilized in producing chemicals not otherwise possible.
injection techniques, promises new chemical products because of the excited states
which the atoms and molecules of the feed stock attain.
plasma have a much higher reactivity than do normal atoms.
molecules collide, either a first or second kind of collision is distinguishable.
In collisions of the first kind, kinetic energy is changed into excitation energy
as shown by:
Instead of exhibiting a barrier
A study of reactant
Excited atoms in flowing
When two atoms or
* A + B + kinetic energydA + B
In collisions of the second kind, a reversal process is given by:
* A + B 4 A + B + kinetic energy
23
Other processes where an atom or molecule gives up exc i ta t ion energy (e) by
co l l i s ion a r e given by;
A + B * j A * + B
and
AB+A + B
An example of co l l i s ions of the second kind is exhibited by mercury (Hg).
a number of Hg at- have been brought i n to an excited s t a t e , 3P1, by i r r ad ia t ion
of Hg vapor with the 2537 A resonance level spec t ra l l ine, the Hg vapor reradiates
the same spec t ra l l ine. I f thall ium vapor is added t o the Hg vapor, i t a l s o
reradiates from about the same energy level . The co l l i s ion process takes place
as follows
When
0
Now i f hydrogen (H2) is introduced, quenching of the resonance rad ia t ion occurs
with the poss ib i l i t y of chemical react ion taking place with the following products
formed :
3 1
3
(2)
(3)
Hg ( P1) + H2,+Hg ( So) + H + H + k ine t i c energy
& ( PI) + H2-)HgH + H + k ine t i c energy
The second and th i rd reactions would not occur a t ordinary temperatures without
the presence of the excited state of mercury.
24
A discussion of problems related to the use of arc plasma in the
chemical industry cannot be concise because of its embryonic stage of development.
The field of plasma chemistry is not developed to the extent of ordinary chemistry
but a basis for it is established in the field of atomic.physics where the kinetics
of excitation and ionization reactions have been and are being investigated.
instances where thermal equilibrium exists, the composition of a given plasma
can be calculated. The interest and the use of plasma chemistry in chemical
processing will continue to increase as this discipline becomes established.
In
25
REFERENCES
1.
2,
3,
4.
5.
6,
7,
8 ,
Maniero, D. A., Kienast, P. F., Hirayama, C:
High-Temperature Chemical Processing, Westinghouse Engineer, May, 1966.
Electric Arc Heaters for
Peters, T.: Temperature und Strahlungsmessungen an wasserstabilisierten
Hochdruckbogen," Z. fur Physik, Bd. 135, 573-592, 1953.
Kivel, Be Bailey, KO :
AVCO Research Laboratory, Everett, Masso Res. Rep. 21.
"Tables of Radiation from High-Temperature Air,
Wallner, L. E., Czika, Joseph Jr.: Arc-Jet Thrustor For Space Propulsion
NASA TND-2868, 1965.
Eschenbach, R. C., Skinner, G.M.: Development of Stable High Pressure
Arc Heaters for a Hypersonic Wind Tunnel, WADD Technical Report 61-100,
July 1961.
Rosenbaum, B. M., and Levitt, Leo: Thermodynamic Properties of Hydrogen
from Room Temperature to 100,OOO°K, NASA Tn-D-1107, 1962,
John, R. R., Debolt, H., Hermann, M. et ale: Theoretical and Experimental
Investigation of Arc Plasma-Generation Technology ADS-2DR-62-729, Part 1.
Stine, Howard, A., and Watson, Velvin, R.: The Theoretical Enthalpy
Distribution of Air in Steady Flow Along the Axis of a Direct Current
Electric Arc, NASA Tn-D-1331, 1962.
26
9. Jedlicka, J. R., Stine, N. H.: Axial Flow Through the Wall-Constricted
Direct Current Arc-Comparison of Theory and Experiment, IEEE, Transactions
On Nuclear Science, Vol. NS-11, p. 104-108, Jan. 1964.
10. "Plasma Jet Technology," NASA SP-5033, Oct. 1965.
11. Hirayama and D. A. Maniero:
Given at 151st National Meeting, ACS, Dive Fuel Chemistry, Vol. 10, No. 1,
Mar. 22-31, 1966, Pittsburgh, Pa.
A New Rotating-Arc Heater for Chemical Processing,
12. Hess, R. V.: "Fundamentals of Plasma Interaction with Electric and Magnetic
Fields, NASA University Conference, 1962.
27
F i g . 1 Marc 30 Arc Heater
Test NO55 Exhaust Flame
January 31, 1965
28
Fig. 2 Helium Plasma Jet Operating in an Ambient Vacuum, Input Power 50 W.
29
100
10
1.
0.1
10 atm
Curve 578653-A
0.0
Hydrogen flw Anode
Cathode
/ T h r n d 1 1 I I I I V Q I
Unconstricted
__I)
_II) Exhaust
Fig. 4- An unconstricted arc
31
Dwg .85 2A 089
. n . Anode I I. .A ._ -- - f I -... nyaroqen TIOW b h _I)
___) /'
., \I
-
Exhaust
( a ) Constricted arc
1 Anode -
Exhaust
(b) Constricted arc with mixing chamber
Fig.5- Constricted arc (a) without and (b) with mixing chamber
32
Dwg .85 a 0 9 0
1
a Feed Gas - - ____) - - - - - Exhaust
4
I Throat
Toroi da I Electrodes V
ArcSpot . on Electrode
Arc
Current E I ect rode
~ rode
Fig. 6- Rotating arc heater
33
Dwg. 852A910
Air I n l e t -,
Field Coi I ' Electrode
Fig. 7-High voltage arc heater schematic diagram WADD TR 61-100
34
Dwg .4508260
Anode \ \ Hydrogen flow -
0
/I, l a r c Cathode -- - Q r radial component of heat transfc
/ ' ?r by conduct ion \
Mechanisms of heat t ransfer in core-flow model
Hydroqen \ Anode /
-- -- -- - ---- ----_
Qr heat transferred to inner flow by arc flow
/ Q, heat transferred to outer flow by wall
Regions of flow and energy source for each in-core-flow model
Fig. 9- Core-flow model
36
I
Buffer zone
Fig. IO-Mechanisms of heat transfer in Stine-Watson theory
37
Curve 5786554 1.5
1.0
Experimental data - St i n e- Wat son theoretical c u rve -
- .5
0 .08 . 16 .24 .32 Distance parameter, z/zo
Fig. 11- Variation of enthalpy parameter with dimensionless distance parameter. Comparison of experiment with Stine-
Watson model for nitrogen.
-
Dimensionless distance parameter, z/zo
Fig. 12- Variation of arc efficiency with dimensionless distance parameter. Comparison of experiment with Stine-Watson model
I '
Curve 576179-8 1
2000 3000 4ooo Temperature, O K
Fig.13-Calculated C t 2H2 as a function of temperature (Total P = 1 atm)
39
r- L. 0 c, ru -
40
aJ N N 0 c
-
I IA
41
Curve 578656-A , ----- Heat flow paths
I sot her ms #
Heat to incoming qas (- 3 kW)
Anode zone heating (- 3 kW) Convection
/
heating (- 3 kW)
B 1600 1700 1800 E
2200
2 100
I A 1900 2000 Radiation (-- 3 kW)
Fig. 16- Engine isotherms and heat flow paths for 30-kilowatt arc-jet engine
42 NASA-Langley, 1967 - 25 CR-813
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